The present invention relates to a scanning optical system with an improved scanning efficiency.
For achieving a high-speed scanning the use of a polygonal mirror as a deflector or scanning device is commonly known. Such polygonal mirror, of an elevated precision, becomes more difficult to manufacture and requires a high manufacturing cost as the number of reflecting surfaces increases. On the other hand a polygonal mirror of a fewer number of reflecting surfaces for reducing said difficulties will result in a scanning function with an inferior scanning efficiency. For example, in a deflector with four deflecting surfaces such as a tetragonal mirror, each surface is capable of deflecting a stationary incident beam over 180.degree.. However, the scanning lens provided between the deflector and the scanned surface is usually capable of covering a half-angle of only 20.degree.-30.degree.. Thus, even if the beam is deflected over 180.degree., the deflected beam can only partly contribute to the scanning, leading to a small proportion of effective scanning time and therefore a low scanning efficiency.